Magnetic inks are known formed of magnetic material dispersed in a liquid carrier.
The magnetic material is typically magnetite (Fe.sub.3 O.sub.4), .nu.--Fe.sub.2 O.sub.3 and the like. Other magnetic material based on cobalt, chromium dioxide, and the like is contemplated by the present invention. The magnetic material, in extremely finely divided form of the order of submicron size, is more or less permanently suspended in a liquid carrier with the aid of dispersing agents, surfactants, and the like to form a colloidal magnetic fluid, typically referred to as a ferrofluid.
The liquid carrier employed is usually a non-aqueous solvent, often an organic solvent of the non-polar type. Examples of non-aqueous solvents usable in the preparation of ferrofluids are aliphatic hydrocarbons, such as heptane, decane, mineral oil, kerosene, and the like, halogenated hydrocarbons such as carbon tetrachloride, trichloroethylene and the like, aromatic solvents such as benzene, toluene and the like, silicone oils, etc.
The dispersing aid is included in the ferrofluid formulation to prevent aggregation of magnetic material particles in the non-aqueous solvent, which could lead to flocculation and deposition out of suspension of magnetic material. The dispersing aid, which may be one or more surfactants, wetting agents and the like, is applied to coat the surfaces of the individual magnetic particles so as to form a coating around the individual magnetic particles to prevent agglomeration or flocculation due to attraction therebetween. Aliphatic carboxylic acids having about 8 to 24 carbon atoms are known as dispersing aids for magnetic material, such as magnetite, to be colloidally suspended in non-aqueous solvents. In this regard, see U.S. Pat. Nos. 3,531,413 and 3,764,540.
For certain applications of magnetic inks it is desirable to employ water as the carrier fluid for the magnetic material in place of the more commonly used non-aqueous solvents. The present invention is directed to the preparation of colloidal suspensions of magnetic material, such as magnetite, in an aqueous medium, and the ferrofluid so produced.
More particularly, the present invention is directed to improvements in aqueous based ferrofluids wherein anionic, cationic and/or non-ionic surface active agents are employed as suspending and dispersant aids for the magnetic material. Preferred surface active agent combinations are as disclosed in commonly-assigned copending application Ser. No. 507,850, filed Sept. 20, 1974, now U.S. Pat. No. 3,990,981 by Kovac et al, the entire disclosure which is incorporated herein by reference. The copending application is directed to rendering finely divided magnetic particles water dispersible through the aid of a combination of one or more non-ionic wetting agents and one or more cationic surface active agents.
It has been determined that aqueous magnetic inks involving the use of anionic, non-ionic or cationic surface active agents present certain problems when employed in magnetic jet ink applications. More specifically, problems have been encountered with respect to filterability, evaporation rate and drying time of the aqueous based ink.
U.S. Pat. No. 3,846,141 discloses, as a humectant system for aqueous inks containing water soluble dyes, a mixture of lower alkoxy triglycol with at least one of a polyethylene glycol, a lower alkyl ether of diethylene glycol and glycerol.
U.S. Pat. No. 1,404,345 discloses the addition of glycerin to an aqueous colloidal suspension of ink pigment to control drying of the ink.
Accordingly, it is an object of the present invention to provide a method for forming a colloidal suspension of magnetic particles, particularly magnetite, in water.
Another object of this invention is to provide an aqueous based magnetic ink characterized by good filterability, fast drying on paper coupled with slow evaporation of the ink itself and permanence of indicia on the printed substrate.
A further object of this invention is to provide a specific combination of dispersing aids and additives which can be employed to produce the abovedescribed aqueous based magnetic ink.